Trunk pans, rear rails and body mounts seem to be the most common place for Regals to rust out. If it has T-Tops, the windshield pillars, roof at top of windshield and as a result front floor pans rust out. Slowly drive it through a puddle and make sure there are only two tire tracks on the other side. Listen for engine noises beyond injectors clicking. Look down the oil filler neck with a flashlight and look for signs of oil contamination. Pull the trans dipstick and smell it for burnt fluid once warm. When it is cold, let it idle with the radiator cap off and look for bubbles. If you are able, do a compression test (not likely to happen but you can ask) and look at the plugs for rich or lean condition. Make sure it starts on the first few revolutions of the engine. No lights should be showing on the instrument cluster. Remember it's a used car and you will need to maintain it and probably put some money in it to get it to your liking.